The present invention generally relates to a liquid crystal polarization reorientation cell and, in particular, relates to one such cell having means for establishing a magnetic field in the liquid crystal material.
The use of optical fibers as a telecommunication media has numerous advantages compared to currently used telecommunication media. For example, optical fibers can carry a broader bandwidth signal, and hence can convey larger quantities of information over the same period of time than existing media.
Further, light waves are shorter than conventional microwaves commonly used in existing telecommunication systems, and thus, a substantial reduction in the physical size of components is achievable. Such a reduction in size further results in a cost reduction for materials, packaging, and manufacturing. Still further, optical fibers exhibit little or no electromagnetic or radio-frequency interference, thus resulting in a negligible impact on the surrounding environment. In addition, optical fibers are relatively insensitive to electromagnetic or radio-frequency interference from surrounding devices and systems.
To be viable, every telecommunication system must include some means for controllably redirecting a signal, or at least a portion thereof, to, or from, a transmission media or between one, or more, such mediums. In the case of an optical telecommunication system, the preferred means is an optical switch. Currently optical switches are generally mechanical in nature.
However, mechanical switches generally require relatively high driving power and are subject to wear, abrasion and fatigue. As a result, mechanical switches are also prone to failure after repeated use. In addition, since a rather small optical fiber is usually displaced from alignment with one port fiber into alignment with another port fiber, mechanical switches can readily become expensive due primarily to the very small tolerances required to ensure the proper alignment between the various fibers thereof.
Liquid crystal optical switching devices have been proposed as one alternative to mechanical switches. Typical of early designs of such liquid crystal optical switches are those described and discussed in U.S. Pat. No. 4,201,422 issued to McMahon et al. on May 6, 1980, U.S. Pat. No. 4,278,327 issued to McMahon et al. on July 14, 1981 and U.S. Pat. No. 4,385,799 issued to Soref on May 31, 1983. Therein, plurality of liquid crystal switches are described wherein optical fibers are attached to the side angled surfaces of a pair of opposing trapezoidal prisms. The trapezoidal prisms are arranged with the major bases thereof parallel and liquid crystal material is disposed therebetween.
Although the optical switches described in these patents have some advantages over mechanical switches, particularly no moving parts, such liquid crystal switches are both difficult and expensive to manufacture since all of the surfaces of the trapezoidal prisms must not only be optically flat, but also be fixed at a precise angular relation to each other. Consequently, the liquid crystal optical switch designs described in these patents are presently impractical for optical communication systems, that frequently require hundreds, if not thousands, of optical switches.
More recently, less expensive more easily manufactured liquid crystal switches have been developed. For example, those described and discussed in U.S. Ser. Nos. 795,138; 795,148, 4,749,258, 795,149, 4,711,529, abandoned 795,150, 795,151, 4,737,019, 795,152, 4,720,174, 795,154; 795,155, 4,720,171, 795,156, 4,720,172, 795,157, 795,296 all filed Nov. 5, 1985 and assigned to the assignee hereof. The designs described therein overcome many of the difficulties present in the earlier designs, and thus, serve as a basis for the relatively inexpensive manufacture of liquid crystal optical switches.
However, to date substantially all of the liquid crystal switch designs have included one, or more, electrodes to establish an electromagnetic field in, or across, the liquid crystal material to effect the switching or beam splitting functions thereof.
The electrodes, in general, are manufactured thin enough that they are transparent to the light incident upon the switching device. Typically, such electrodes include a thin transparent layer of Indium Tin Oxide. However, the index of refraction of such a conductive material is on the order of about 1.9 whereas the index of refraction of typical nematic liquid crystal materials is on the order of about 1.6. Such a mismatch can result in severe reflections at the interface therebetween, thereby reducing the efficienty of the light transferred due to spurious reflections and other deleterious effects.
Such a mismatch between adjacent materials can be substantially overcome by implementing techniques described in the above referenced U.S. patent application Ser. No. 795,150 filed on Nov. 5, 1985 and entitled LIQUID CRYSTAL CELL FOR USE IN AN OPTICAL SWITCH.
As described therein, intermediate layers having a gradient index of refraction can be formed between the electrode and the liquid crystal material and, in addition, between the transparent member, i.e. glass that also usually has index of refraction of about 1.6, and the ITO electrodes. Although this technique can be implemented inexpensively, and low lost devices can be fabricated, this technique can be a time consuming process during the manufacture of such devices. Further, care must be taken to avoid impurities in the compensatory layers.
Consequently, a liquid crystal device that can avoid the need to compensate for such general reflection, as well as for localized impurities, but effect the desired switch is desirable to more readily derive the benefits of optical fiber signalling systems.